This invention relates to a fuel cell system and more particularly to a system having a plurality of cells which consume an H2-rich gas to produce power.
Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid polymer electrolyte membrane having the anode catalyst on one of its faces and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distributing the fuel cell""s gaseous reactants over the surfaces of the respective anode and cathode catalysts. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A plurality of individual cells are commonly bundled together to form a fuel cell stack and are commonly arranged in series. Each cell within the stack comprises the membrane electrode assembly (MEA) described earlier, and each such MEA provides its increment of voltage. A group of adjacent cells within the stack is referred to as a cluster. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113, assigned to General Motors Corporation.
In PEM fuel cells, hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2), or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and mixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies are relatively expensive to manufacture and require certain conditions, including proper water management and humidification, and control of catalyst fouling constituents such as carbon monoxide (CO), for effective operation.
For vehicular applications, it is desirable to use a liquid fuel such as an alcohol (e.g., methanol or ethanol), or hydrocarbons (e.g., gasoline) as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store onboard and there is a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished within a chemical fuel processor or reformer. The fuel processor contains one or more reactors wherein the fuel reacts with steam and sometimes air, to yield a reformate gas comprising primarily hydrogen and carbon dioxide. For example, in the steam methanol reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide. In reality, carbon monoxide and water are also produced. In a gasoline reformation process, steam, air and gasoline are reacted in a fuel processor which contains two sections. One is primarily a partial oxidation reactor (POX) and the other is primarily a steam reformer (SR). The fuel processor produces hydrogen, carbon dioxide, carbon monoxide and water. Downstream reactors may include a water/gas shift (WGS) and preferential oxidizer (PROX) reactors. In the PROX carbon dioxide (CO2) is produced from carbon monoxide (CO) using oxygen from air as an oxidant. Here, control of air feed is important to selectively oxidize CO to CO2.
Fuel cell systems which process a hydrocarbon fuel to produce a hydrogen-rich reformate for consumption by PEM fuel cells are known and are described in U.S. patent application Ser. Nos. 08/975,422 and 08/980,087, filed in November, 1997, now U.S Pat. Nos. 6,232,005, and 6,077,620, respectively and U.S. Ser. No. 09/187,125, filed in November, 1998, now U.S. Pat. No. 6,238,815 and each assigned to General Motors Corporation, assignee of the present invention; and in International Application Publication Number WO 98/08771, published Mar. 5, 1998. A typical PEM fuel cell and its membrane electrode assembly (MEA) are described in U.S. Pat. Nos. 5,272,017 and 5,316,871, issued respectively Dec. 21, 1993 and May 31, 1994, and assigned to General Motors Corporation.
Efficient operation of a fuel cell system depends on the ability to effectively control gas flows (H2 reformate and air/oxygen) to the fuel cell stack not only during start-up and normal system operation, but also during system shutdown. During the shutdown of a fuel cell system that generates hydrogen from liquid fuel, the anode CO emissions increase and can degrade the stack. Accordingly, a primary concern during shutdown is diverting the gas flows of H2and air/oxygen around or away from the fuel cell stack and disposing of the excess H2. The H2 and air flows being diverted from the stack during shutdown must also be kept separate to avoid creating a combustible mixture in the system. The stack must also be protected from prolonged (e.g., greater than five seconds) pressure differentials which could rupture the thin membranes in the membrane electrode assembly (MEA) separating the anode and cathode gases.
Fuel cell systems, in particular those used in vehicular applications, are often used to generate start-up and transient heat for the fuel processor. The combustor is fueled by the anode and cathode effluents, supplemental hydrocarbon fuel for start-up and high demand situations, and excess H2 from the fuel processor. The combustor is also useful for burning off residual stack effluents and processor H2 during system shut-down. During normal system operation, the combustor typically runs at a constant temperature, for example around 600xc2x0 Celsius in an exemplary vehicle propulsion system application. It is important at all times to prevent the combustor from overheating, as the resulting degradation would require an expensive replacement and would interfere with the operation of the system as a whole. The combustor therefore generally receives a continuous air flow from the system air supply. Air flow to the combustor must be maintained during shutdown to prevent overheating as the combustor burns off residual gases.
The cooling of the combustor therefore competes with the shutdown objectives of gas flow diversion and residual H2 combustion. Especially where the air supply to the system generally supplies both the combustor and the cathode inlet of the fuel cell stack, the diversion and venting of air from the cathode inlet must not even temporarily deprive the combustor of sufficient airflow for cooldown.
During normal shutdown of the system in which time is not a factor, the competing demands of gas flow diversion and combustor cooldown are relatively easy to offset and satisfy. However, during rapid shutdown, carbon monoxide emissions at the stack anode and pressure differentials at the cathode need to be dissipated in a few seconds. At the same time, sufficient air flow must be maintained to the combustor for the lengthier cooldown period. The coordinated diversion and venting of the gas flows with respect to both the fuel cell stack and combustor becomes difficult.
In one aspect, the invention provides a venting methodology for staging the diversion and venting of reformate H2 and air relative to the fuel cell stack, the combustor, and one or more vents. This staged venting protects the stack from degradation due to CO and due to high pressure differentials, and protects the combustor from overheating. In another aspect, the invention further provides a currently-preferred valving and control scheme for carrying out the venting methodology.
In a fuel cell system in which the fuel cell stack and the combustor are supplied with H2 and air, respectively, by a common H2 supply and a common air supply, and each of the H2 supply and air supply is provided with a bypass valve which supplies both the stack and the combustor during normal system operation but which bypasses the stack to the combustor during shutdown, the stack anode inlet is instantaneously vented as the bypass valves are commanded to close. The air flow ratio is slowly shifted between the cathode inlet and the combustor by the air supply bypass valve until the air flows almost entirely to the combustor. The cathode inlet is vented at a point during the air flow ratio shift at which venting will not significantly affect the flow of cooling air to the combustor, but before the pressure differential between the cathode and anode inlets can degrade the membranes in the stack.
According to another feature of the invention methodology, the H2 supply path to the combustor is vented simultaneously with the stack anode inlet.
According to another feature of the invention methodology, the H2 from the anode inlet and the air from the cathode inlet are vented through separate vents to prevent the creation of a combustible mixture in the system. Both of the H2 and air vents preferably vent to atmosphere although other arrangements (adsorbers, holding tanks) might be useful for certain applications.
According to another aspect of the invention, the invention methodology is carried out by fast-acting vent valves provided in the flow path of the H2 bypass valve to the anode inlet; in the H2 supply path from the H2 bypass valve to the combustor; and in the air supply path between the air bypass valve and the cathode inlet. The vent valving for carrying out the invention methodology may comprise existing valves and a fuel cell system controlled according to the invention methodology during a rapid shutdown, or may comprise single-purpose valving added to an existing fuel cell system. Control of the vent valving can be through a dedicated controller comprising any suitable microprocessor, microcontroller, personal computer, etc. which has a central processing unit capable of executing a control program and data stored in the memory. The controller may additionally comprise an existing controller in a fuel cell system.